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Newsgroups: comp.lang.ada
Date: Tue, 5 Nov 2019 07:10:43 -0800 (PST)
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Subject: Re: How to best make a custom range?
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:57485
Date: 2019-11-05T07:10:43-08:00
List-Id: <comp.lang.ada>

On Tuesday, November 5, 2019 at 7:02:37 AM UTC-7, Andrew Shvets wrote:
> This is what I have now.
>=20
> subtype DNA_Chars is Character range =E2=80=98A=E2=80=99 .. =E2=80=98Z=E2=
=80=99;
> subtype DNA_Symbols is Character with Static_Predicate =3D> DNA_Symbols i=
n =E2=80=98 =E2=80=98;
> subtype DNA_Char is Character with Static_Predicate =3D> DNA_Char in DNA_=
Chars | DNA_Symbols;
>=20
> And this is the warning that I get (one of many, but I can=E2=80=99t copy=
 and paste):
>=20
> warning: in instantiation at a-nudira.adb:54
> type =E2=80=9CResult_Subtype=E2=80=9D has predicates, attribute =E2=80=9C=
First=E2=80=9D not allowed

OK, the problem here is that the Ada language *does* discriminate between s=
ubtypes with predicates and those without -- mostly because there are argum=
ents about how such attributes should behave. Things like if we have:
   TYPE Digits is range 0..9;
   SUBTYPE Odds is Digits with Static_Predicate =3D> Odds in 1|3|5|7|9;
what should Odds'Pred(1) be? 0? Constraint_Error?
what about Odds'Pos(1) should it be 0, the first item of the subtype? Or 1,=
 the position in the parent-type?